The present invention relates to electrical filters for passing a band of frequencies, and more particularly to wide-bandwidth monolithic crystal filters.
According to the prior art, the passband of a crystal filter may be increased by parallel connecting a number of two-pole crystal filters which each have different center frequencies. A crystal filter of this type is described in U.S. Pat. No. 3,697,788. Other techniques which have provided somewhat greater passbands include reducing the separation between the input and output electrodes while coupling along the X crystallographic axis of the quartz substrate; utilizing single-phase coupling between rectangular electrodes instead of circular electrodes; and combining both of the foregoing techniques with light plateback. However, none of the foregoing techniques have provided a practical means for producing wide-bandwidth monolithic crystal filters that have relatively large passbands, for example, 30 KHz, and relatively high filter center frequencies, for example, 45 MHz.